The present invention relates to a time display method and a time display apparatus for use in a continent where a plurality of time zones which are different in delay time (a difference in time) from the standard time are present.
In a continent such as North America, a plurality of time zones which are different in delay time from the Greenwich standard time are present. FIG. 10 is an explanatory view of time zones in the U.S.A. mainland, where four time zones are present. A first time zone is a time zone (Eastern) with a 5-hour delay from the standard time, which is an Eastern zone on the right side of a boundary line connecting AAxe2x80x2. A second time zone is a time zone (Central) with a 6-hour delay from the standard time, which is a Central zone surrounded by a boundary line connecting BBxe2x80x2 and the boundary line connecting AAxe2x80x2. A third time zone is a time zone (Mountain) with a 7-hour delay from the standard time, which is a Mountain zone surrounded by a boundary line connecting CCxe2x80x2 and the boundary line connecting BBxe2x80x2. A fourth time zone is a time zone (Pacific) with an 8-hour delay from the standard time, which is a Pacific zone on the left side of the boundary line connecting CCxe2x80x2.
When a plurality of time zones are present as described above, every time a train or a long-distance truck crosses these time zones and moves into a new time zone, a time display must be corrected on the basis of the new time zone. For example, in the conventional navigation apparatus mounted on the vehicle provided with a time display function, a user suitably sets an applicable time zone (Eastern, Central, Mountain, or Pacific) manually, and the navigation apparatus calculates the present time on the basis of a difference in time in the time zone set to display it. However, the operation for setting a time zone every time poses a cumbersome problem. Because of this, U.S. Pat. No. 5,842,146 proposes a method for automatically discriminating a time zone to which a vehicle belongs and calculating the present time on the basis of a difference in time in the time zone to display it. That is, the proposed method is to find the present position (longitude and latitude) of a GPS (Global Positioning System) vehicle, and find the time zone where the vehicle is present from the vehicle""s present position and geographical information of the time zone. However, since it is necessary to geographically accurately specify the boundary lines AAxe2x80x2, BBxe2x80x2, and CCxe2x80x2 of the time zones in FIG. 10, the proposed method has the problem that a large quantity of memory is required, and in addition considerable processing is necessary to discriminate the appropriate time zone. Therefore, a method is contemplated in which a boundary line of a time zone is approximated by a single straight line, but an error occurs in the time display in the vicinity of the boundary line.
From the foregoing, it is an object of the present invention to provide a time display method and apparatus without the necessity of geographically specifying the boundary lines AAxe2x80x2, BBxe2x80x2, and CCxe2x80x2 of the time zones.
It is a further object of the present invention to provide a time display method and apparatus capable of displaying time taking a time zone and daylight saving into consideration by simple processing.
According to the present invention, the aforementioned object is achieved by a time display method comprising: (1) storing the delay time from the standard time for every state, or if a state belongs to a plurality of time zones, storing the delay time of each time zone for the state and boundary information for specifying a region belonging to each time zone; (2) monitoring the present position of a vehicle and detecting the state in which the vehicle is present; (3) referring to the stored information, and if the state in which the vehicle is present belongs to only one time zone, performing a time display on the basis of the delay time corresponding to the time zone; and (4) if the state in which the vehicle is present belongs to a plurality of time zones, finding a region in which the vehicle is present on the basis of said boundary information and the present position of the vehicle, and performing a time display on the basis of the delay time corresponding to the time zone to which the region belongs.
Further, according to the present invention, the aforementioned object is achieved by a time display method comprising: (1) storing information on whether or not daylight saving is enforced in every state; (2) discriminating whether or not a state in which a vehicle is present is a state where daylight saving is enforced by referring to the stored information; (3) if the state is a state where daylight saving is not enforced, performing a time display on the basis of the delay time corresponding to the time zone of the state; and (4) if the state is a state where the daylight saving is enforced, judging whether the time display is performed on the basis of either summer time or winter time, and performing the time display on the basis of the delay time from the standard time in summer time or winter time of the state.
Further, according to the present invention, the aforementioned object is achieved by a time display apparatus comprising: (1) a difference in time information storing means for storing the delay time from the standard time for every state, or if a state belongs to a plurality of time zones, storing the delay time of each time zone for the state and boundary information for specifying a region belonging to each time zone; (2) a vehicle position monitoring means for monitoring the present position of a vehicle; (3) a vehicle presence state detecting means for detecting a state in which the vehicle is present; (4) a present time calculating means for calculating, if the state in which the vehicle is present belongs to only one time zone by referring to said stored information, the present time on the basis of the delay time corresponding to the time zone; and calculating, if the state in which the vehicle is present belongs to a plurality of time zones, the present time on the basis of the delay time corresponding to the time zone to which a region in which the vehicle is present belongs; and (5) a display for displaying the calculated present time.
Further, according to the present invention, the aforementioned object is achieved by a time display apparatus, wherein (1) said difference in time information storing means stores information on whether or not daylight saving is enforced for every state, and (2) said present time calculating means calculates, by referring to the stored information and if the state in which the vehicle is present is a state where daylight saving is not enforced, the present time on the basis of the delay time corresponding to the time zone of the region, and calculates, if the state in which the vehicle is present is a state where daylight saving is enforced, the present time on the basis of the delay time from the standard time in summer time or winter time.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.